Measurement of the Fresnel reflection at the end of an optical fibre provides a very convenient method of determining the refractive index of fluids. The measurement volume is very small, the fibre probe is inert and passive, and the measurement can be made with good sensitivity by fibre reflectometry. The technique has been examined for applications in medicine, where it can employ the dependence of fluid refractive index on a number of parameters of medical interest, such as chemical composition and pressure.
Of particular interest to petrochemical operations is the remote identification of interfaces between immiscible fluids. In this situation, however, the type of fibre probe used in the above work presents problems. Square-cleaved fibres are not suitable as sensors of the passage of an interface between immiscible fluids because of the tendency for droplets of one or the other fluid to adhere to the end surface of the fibre. In most cases such droplets are thick enough to dominate the end reflection so that the transfer of the fibre end to the other fluid is not detectable. This phenomenon is noticeable in such a simple case as the immersion of a fibre in water and its subsequent withdrawal to air. In general, the droplet which forms on the end must be removed or allowed to evaporate in order to observe the Fresnel air reflection from the fibre end.
Tapered optical fibres are known, but optical fibres having the tapers described here for use as fibre optical sensors are not known.
For example, reflections from a 45 degree taper have been used previously to measure gas void fractions in fluids, the taper serving in effect as a corner reflector. Spindler et al, Faseroptischer Sensor zur Messung des ortlichen Gasgehaltes in Flussigkeiten, Tecnisches Messen tm54, pp. 50-55, shows a tapered fibre that is useful to detect the rather large index difference between fluids and gases. At this particular angle the taper acts like a cornercube reflector and sends the light back directly the way it came after exactly two bounces. The taper is so abrupt t hat it cannot function as in the present invention to cause an interaction between the light and the external medium at the cladding boundary. It is useful only for detecting rather large index differences in the external medium, such that for one medium the 45 degree incidence angle is greater than or near the critical angle, while for another medium it is less than the critical angle. The taper is not used to keep the probe clear of contamination. Such a taper appears too steep to be effective for use to enhance the dependence of the reflections on the external refractive index or to remove the problem of adhering droplets, as described in this patent. In the present invention, the tapers are considerably shallower, around 20 degrees, and are preferably formed by drawing down the fibre.
Tapered fibres are also known for connection of two optical fibres to each other or for coupling radiation from a light source into a fibre. However, the purpose of those fibres is different and the advantages of the present fibre would not have been obvious from knowledge of the existence of those tapered fibres.
In particular, fibres tapered for input coupling purposes possess flat or bulbous ends whereas the tapered optical fibres of the present invention can function when the taper extends to a point. The taper angle for the optical fibre sensor described here is determined on a different basis than for a coupler. The angle in the optical fibre sensor of the present invention must be chosen to provide large numbers of cladding boundary reflections without reducing the overall reflection below what can be detected in a particular application. The taper input coupler, on the other hand, maximizes the input coupling of radiation. In general, the tapered optical fibre of the present invention requires smaller taper angles than taper input couplers.
Biconical tapers are also known for coupling light laterally between two or more fibres. These differ from the present invention because the tapers do not terminate in ends, but reverse, so that they expand back to the original fibre size. The reversal of the direction of the light is undesirable in these devices, and the tapers are formed so as to prevent it.
The tapered fibre of the present invention provides several unexpected advantages over these prior art tapers, including that the core taper allows the unbound propagating modes to interact more often at the interface with the external medium and that droplets of another medium tend to adhere away from the interface where the Fresnel reflection of interest occurs. These advantages will now be described in more detail.